photoperiodism

In our class discussion of the photoperiodic control of flowering, after I
presented the experiments showing (by means of night interruption) that
plants are timing the length of the night, rather than the length of the
day, a student asked **why** (in an evolutionary sense) it is the night
that is timed.
Any suggestions out there?
Carl S. Pike (717) 291-3958
Department of Biology FAX (717) 399-4548
Franklin and Marshall College Internet C_PIKE at ACAD.FANDM.EDU
P.O. Box 3003
Lancaster, PA 17604-3003 USA